The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporate microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A detailed description of modern cardiac pacemaker technology is set forth in International Application No. PCT/US85/02010, entitled STIMULATED HEART INTERVAL MEASUREMENT, ADAPTIVE PACER AND METHOD OF OPERATION, assigned to the assignee hereof. The disclosure of that application is incorporated herein by reference.
In order to efficiently perform its function as a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, these contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram and include a P-wave, representing the depolarization of the atria; the QRS wave (sometimes referred to as an R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and is masked out by the much larger QRS-wave on the electrocardiogram.) Thus, it is the P-QRS-T cycle of waves that represents the natural AV synchrony of the heart. These waves, including the time relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation of the heart is being examined.
Multiple-mode, dual-chamber, demand-type, cardiac pacemakers are designed, insofar as possible, to maintain an AV synchrony for damaged or diseased hearts that are unable to do so on their own. This is realized by placing electrodes in both the right atrium and right ventricle of the heart. These electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization or contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead or electrode as is used for sensing. This stimulation pulse causes or forces the desired depolarization and contraction of the indicated chamber to occur. Hence, by first sensing whether a natural depolarization occurs in each chamber, and by second stimulating at controlled time intervals each chamber with an external stimulation pulse in the absence of a natural depolarization, the AV synchrony of the heart can be maintained.
Unfortunately, there are many operating constraints and conditions of the heart that complicate the operation of a demand-type pacemaker. (A demand-type pacemaker is one that provides a stimulation pulse only when the heart fails to produce a natural depolarization on its own within a prescribed escape interval.) For example, there are certain the periods following a depolarization of cardiac tissue (prior to repolarization) when the application of an external electrical impulse is ineffective--that is, it serves no useful purpose, and thus represents an unneeded expenditure of the pacemaker's limited energy. Therefore the application of stimulation pulses during these time periods is to be avoided. Further, it is not uncommon for extraneous electrical signals or noise to be present. These electrical noise signals may be of sufficient amplitude to be sensed by the sensing amplifiers of the pacemaker, which sensing can "fool" the pacemaker into thinking that is has sensed electrical activity associated with a natural depolarization of the heart tissue, when in fact all that it has sensed is noise.
Where signals originating in one chamber of the heart are picked up and sensed by sensing circuits designed to sense signals in the other chamber of the heart, this particular noise problem is identified as "crosstalk." As a specific example, the atrial sense circuits may sense activity that occurs in the ventricle (also known as farfield sensing), or the ventricle sensing circuits may sense activity that occurs in the atrium (most common, and the main problem addressed by this invention). In either event, the pacemaker logic technically has no way to determine whether the sensed signal is a legitimate signal (one that should be acted upon in a prescribed manner) or a crosstalk signal (one that essentially represents noise and should not be acted upon as a legitimate signal).
While it should be noted that crosstalk can originate from several sources, for purposes of this application the most common source of crosstalk is the atrial stimulation pulse which is generated by the pacemaker being cross-coupled to the sensing circuits or electrode of the ventricular channel.
In order to prevent the pacemaker from generating and delivering stimulation pulses during the natural refractory time period of the heart, or from sensing and responding to electrical noise, it is common in the art to include within the pacemaker a timer circuit that defines a refractory period immediately subsequent to the sensing of major electrical activity, or immediately subsequent to the generating of an electrical stimulus. Pacemaker refractory periods have been used to block out noise for a prescribed time interval during a cardiac cycle. For example, during such a refractory period, the pacemaker sense amplifiers may be disabled for a first portion--the absolute refractory period--during which nothing can be sensed. During a second portion--the relative refractory period--the sense circuit can sense activity, but that which is sensed is usually considered to be noise.
Other methods are known in the art for specifically detecting or minimizing crosstalk or cross-coupling between heart chambers or the respective channel circuits or electrodes of the pacemaker. For example, U.S. Pat. No. 4,462,406 of DeCote, Jr. discloses the prevention of crosstalk between atrial and ventricular channels by multiplexing the atrial leads and ventricular leads at about 2 kHz, a rate which is well above the sense amplifier's upper frequency response.
U.S. Pat. No. 4,462,407 of Herscovici et al. prevents crosstalk between atrial and ventricular leads by using separate input/output circuits for the two channels which are powered by respective isolation capacitors. This arrangement is directed primarily to eliminating crosstalk originating within the pacemaker circuits.
U.S. Pat. No. 4,470,418 of Herscovici et al. reduces interchannel crosstalk in a dual-chamber pacemaker designed for bipolar leads by using a switching circuit that shunts an isolation resistance buffer amplifier in series with the lead electrodes during stimulation. This patent also discloses connecting a differential amplifier in series with a led during sensing, which amplifier is bypassed during pacing.
U.S. Pat. No. 4,586,507 of Herscovici prevents cross-stimulation, a related but not identical problem to crosstalk, between the atrium and ventricle leads during pacing by switchably connecting separate output capacitors in series with the respective leads, which capacitors are isolated from being charged in one channel until after a stimulus has been delivered on the other channel.
These examples of known prior art, while directed to the problem of crosstalk and similar problems, as is the present invention, all depend upon switching the applicable circuits on separate signal channels at different intervals during the heart pacing cycle. Dealing with the problem of crosstalk in such a manner adds to the complexity of the circuitry of the pacemaker, increases the circuit drain on the pacemaker power source, and may degrade pacemaker reliability.